1. Field of the Invention
The present invention relates to a power source circuit incorporating a protection device therein, more particularly, to a power source circuit effectively applied to a printer device such as a wire dot printer or the like.
2. Description of Related Arts
A conventional power source circuit utilized in a printer or the like rectifies an alternate current (A.C.) from an A.C. power source into a direct current (D.C.), generates a high frequency current signal by a switching control circuit, transforms the current signal by a transformer, rectifies it and outputs a D.C. voltage having various values.
In this case, there is provided an alarm circuit in the power source circuit which detects an occurrence of various abnormal operations when a printer is activated, and when the abnormal operation occurs at an external circuit which is connected with the output terminal of the power source circuit, then the alarm circuit transmits an alarm signal to the switching control circuit to suspend the operation of the switching control circuit.
However, there arises a problem in the conventional power source circuit having such a protection function that when an user of the printer turns on a power source switch again just after turning it off after an alarm detection circuit detects an excess current and turns off the switching control circuit, the excess current again flows so as to cause high power source being applied to the external circuit. If an attempt is made to have the alarm detection circuit detect the excess current, the alarm detection circuit requires a long period of time until it detects the high power source being applied to the external circuit because the alarm detection circuit is designed by considering the current and the duration time of the current under a normal operation of the printer.
FIG. 7 is a time chart of current when the printer is operated; and FIG. 8 is a time chart of current when the power source switch of the printer is turned on.
As shown in FIG. 7, under the normal operation of the printer, the current I.sub.1 flows during the time .DELTA.t.sub.1 necessary to print for one line and the current less than I.sub.1 flows during time necessary for a line feed.
Also as shown in FIG. 8, when the power source switch of the printer is turned on, a capacitor in the external circuit provides the rush current I.sub.2 during the time .DELTA.t.sub.2. Since the alarm detection circuit is designed by considering the current I.sub.1, the time .DELTA.t.sub.1 during which the current I.sub.1 flows, the rush current I.sub.2, and the time .DELTA.t.sub.2 during which the rush current I.sub.2 flows, a preset time t.sub.s with which the alarm detection circuit detects the excess current has been set with a long period of time. Accordingly, it takes, for example, approximately four seconds from turning off the switching control circuit to suspending the supply of the power source voltage to the external circuit. Therefore, there arises a problem that the high power source application to the external circuit due to the flow of the excess current may magnify damages of the output circuit.